The present invention relates to a method of supplying fuel to a combustion chamber of a turbojet engine having both a low power head with a plurality of low power fuel injectors and a high power head having a plurality of high power fuel injectors.
Aircraft turbojet engines are required to operate in various modes both at very high power output and at relatively low power output. Turbojet engines utilized in military aircraft must also minimize infra-red emissions in order to prevent detection of the presence of the aircraft during any mode of operation. Accordingly, fume emissions from carbon particles and infra-red emitting fumes from nitrogen oxides must be reduced. The carbon particles and fumes are produced predominantly during high power operations.
Dual head combustion chambers are known to reduce polluting emissions of the turbojet engine especially during high power operations. The high power, or take off, head is optimized for full power operation and feeds a sufficiently lean fuel/air mixture to the combustion chamber to reduce fume production and the formation of large quantities of nitrogen oxides. In low power operation, only the low power head supplies a rich fuel/air mixture to the primary zone of the combustion chamber to ensure flame stability, thereby preventing engine flame-out. The richness of the fuel/air mixture produces large quantities of fume emissions during such low power operations.
Typically, the high power or take-off head and the low power head are radially displaced from each other about the axis of the turbojet engine. This causes a non-homogeneous radial temperature distribution in the gases emanating from the combustion chamber and contacting the turbine blades of the engine. Such non-homogeneous radial temperature distribution diminishes the useful life of the turbine blades.
The high power or take-off head is typically supplied with fuel only beyond 25% of the nominal engine thrust which causes the richness of the fuel/air mixture in the primary zone of the combustion chamber to drop markedly when operation of the high power head is initiated due to the relatively leaner fuel/air mixture than that supplied by the low power head.
It is known to supply a single head, conventional combustion chamber with dual injectors comprising two coaxial tubes feeding fuel to separate zones of the combustion chamber through separate fuel circuits and separate air circuits. Such a system is illustrated in British Patent No. A 2,214,630 which describes a dual injector having two separate fuel circuits, a main feed circuit associated with a first axial swirler to feed a central zone and second feed circuit associated with a second axial swirler to feed an annular zone through channels in the blades of the first swirler. The central zone is operative at low power while the annular zone operates only at high power.
French Patent No. A 2,421,342 also describes a double zone injector wherein the central zone operates only at high power. These documents are silent on the concept of such double zone injectors outfitting a takeoff head of a dual head combustion chamber.